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1
Preheat oven to 350F.
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2
Butter and flour two 9-inch-diameter cake pans with 11/2-inch-high sides.
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3
Combine banana muffin mix, mashed ripe bananas, eggs, vegetable oil, and whole milk in large bowl.
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4
Using electric mixer, beat mixture at medium speed until well blended, about 2 minutes.
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5
Using clean dry beaters, beat egg whites in medium bowl until soft peaks form.
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6
Gradually add sugar, beating until mixture is stiff but not dry.
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7
Fold egg whites into batter in 2 additions.
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8
Divide cake batter between prepared pans.
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9
Bake cake batter until tester inserted into center comes out clean, about 30 minutes.
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10
Cool cakes in pans on racks 10 minutes.
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11
Turn out cakes onto racks and cool completely.
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12
Stir chopped white chocolate in top of double boiler over barely simmering water until almost melted.
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13
Remove from over water; stir white chocolate until smooth.
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14
Cool to lukewarm, about 4 minutes.
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15
Beat cream cheese and unsalted butter in large bowl until fluffy.
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16
Gradually beat in lukewarm white chocolate, then powdered sugar, vanilla extract, and lemon juice.
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17
Place 1 cake layer on platter.
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18
Spread with 1 1/3 cups white chocolate-cream cheese frosting.
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19
Thinly slice 2 bananas.
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20
Arrange banana slices over frosting.
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21
Top with second cake layer.
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22
Spread remaining frosting over top and sides of cake.
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23
Press toasted chopped walnuts into frosting on sides of cake.
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24
Refrigerate cake until frosting is set, at least 2 hours and up to 1 day.
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25
Slice remaining banana; arrange slices atop cake and serve.
